The present invention relates to a method of producing a dielectric ceramic composition having resistance to reduction, a specific permittivity of 1000 or more, a capacitance-temperature characteristic satisfying the X8R characteristic (within xe2x88x9255 to 150xc2x0 C., within xcex94Cxc2x115%) of the EIA standard, small dielectric loss, high permittivity, high insulation resistance and an excellent high temperature accelerated lifetime characteristic.
A multilayer ceramic capacitor is widely used as a compact, large capacitance, highly reliable electronic device, and the number used in one electronic device reaches large. In recent years, along with devicees becoming compact and high in performance, demands for still more compact, larger capacitance, lower price and more reliable multilayer ceramic capacitor have been getting furthermore stronger.
As a dielectric ceramic composition having a high permittivity and a flat capacitance-temperature characteristic, there is known a composition wherein BaTiO3 is a main component and Nb2O5xe2x80x94Co3O4, MgOxe2x80x94Y, a rare-earth element (Dy, Ho, etc.), Bi2O3xe2x80x94TiO2, etc. are added thereto. The temperature characteristic of the dielectric ceramic composition containing BaTiO3 as a main component has difficulty in satisfying the R characteristic (xcex94C=within xc2x115%) of the capacitance-temperature characteristic in a high temperature region of 130xc2x0 C. or more because the Curie temperature of BaTiO3 is near 120xc2x0 C. Therefore, Batio3 based material of high permittivity could fulfill only the X7R characteristic (xe2x88x9255 to 125xc2x0 C., xcex94C=within xc2x115%) of the EIA standard.
In recent years, a multilayer ceramic capacitor has come to be used in a variety of electronic devices, such as an engine electronic control unit (ECU) installed in an engine room, a crank angle sensor, anti-lock brake system (ABS) module in vehicles. Since these electronic devices are for stabilizing engine control, drive control and brake control, temperature stability of the circuit is required to be good.
An environment in which these electronic devices are used is considered that the temperature becomes about xe2x88x9220xc2x0 C. or lower in winter in cold districts and the temperature rises up to about +130xc2x0 C. or more after starting the engine in summer. Recently, the trends are to reduce wire harnesses connecting the electronic device with an apparatus as its object to control and the electronic device is provided outside a vehicle in some cases, so the environment has been getting more severe for the electronic device. Accordingly, a conventional dielectric ceramic composition having the X7R characteristic is unable to cope with such an application.
Also, as a capacitor material for temperature compensation excelling in the temperature characteristic, (Sr,Ca)(Ti,Zr)O3 base, Ca(Ti,Zr)O3 base, Nd2O3xe2x80x942TiO2 base, La2O3xe2x80x942TiO2 base, etc. are generally known, however, since these compositions have a very low specific permittivity (generally, not more than 100), it is substantially impossible to produce a capacitor having a large capacitance.
On the other hand, in a dielectric ceramic composition containing BaTio3 as a main component, there has been a proposal to shift the Curie temperature to a high temperature side by substituting Ba in BaTiO3 by Bi, Pb, etc. so as to satisfy the X8R characteristic (the Japanese Unexamined Patent Publication Nos. 10-25157, 9-40465). Also, there has been proposed to satisfy the X8R characteristic by selecting a composition of BaTiO3+CaZrO3+ZnO+Nb2O5 base (the Japanese Unexamined Patent Publication Nos. 4-295048, 4-292458, 4-292459, 5-109319 and 6-243721). In any of these composition bases, however, since Pb, Bi and Zn easy to be evaporated and scattered are used, firing in an oxidizing atmosphere, such as in an air, becomes premise. As a result, there is a problem that inexpensive Ni or other base metals cannot be used as internal electrodes in the capacitor and expensive rare metals like Pd, Au, Ag, etc. have to be used.
To solve the problem, the present inventors have proposed a dielectric ceramic composition wherein Ni or Ni alloy can be used as internal electrodes in a capacitor (the Japanese Unexamined Patent Publication Nos. 10-2206, 11-206291 and 11-206292).
According to these techniques, by making rare-earth elements, such as Sc, Er, Tm, Yb, Lu, etc., contained in the dielectric ceramic composition, it becomes possible to shift the Curie temperature to the high temperature side and to flatten the capacitance-temperature change rate at the Curie temperature or higher, reliability (high temperature load lifetime, capacitance time change, etc.) can be improved.
However, a second phase containing a rare-earth element as a main component is apt to be segregated in the dielectric when increasing an amount of rare-earth element to be added to the dielectric ceramic composition. Due to the segregation of the second phase, strength of the dielectric was improved, but it was confirmed by the present inventor that the segregated second phase came to have about the same thickness as that of the dielectric layer of the multilayer capacitor depending on the thickness of the dielectric layer, and reliability of the capacitor was liable to be declined. Also, it was confirmed by the present inventors that a product (CR product) of the permittivity and the insulation resistance were apt to decline along with an increase of the amount of the second layer segregated in the dielectric layer.
Also, multilayer ceramic capacitor to be installed in vehicles has made progress in attaining large capacitance and compact size, and the trends are that the thickness of the dielectric layer is getting thinner. Therefore, particularly, in a dielectric ceramic composition wherein rare-earth element is added to dielectric ceramic composition, a technique to control the size and amount of the second phase to be segregated becomes necessary.
To improve reliability of a dielectric ceramic composition having the X7R characteristic, there is a proposal of a method of calcining BaTiO3 as a main material and additives in advance (the Japanese Examined Patent Publication No. 7-118431). In the technique disclosed in the publication, it is considered that the composition of the finally composed dielectric composition becomes (Ba, Mg, Ca, Sr, Zn)(Ti, R)O3+(Ca, Ba)ZrO3+glass. Then, when assuming R=Sc, Y, Gd, Dy, Ho, Er, Yb, Tb, Tm and Lu, calcination of BaTiO3 and the additives is performed so that the mole ratio expressed by (Ba+Mg+Ca+Sr+Zn)/(Ti+Zr+R) comes down to the range of 1.00 to 1.04.
In this method, however, it is considered as premise that alkaline earths (Mg, Ca, Sr, Ba) dissolves in a Ba site and rare-earth elements (R=Sc, Y, Gd, Dy, Ho, Er, Yb, Tb, Tm, Lu) dissolves in a Ti site. Thus, along with an increase of an amount of rare-earth elements to be added, it is necessary that an amount of the alkaline earth to be added is inevitably increased. Also, when the mole ratios in this composition base are specified as above, the high temperature load lifetime (IR lifetime) as the X8R characteristic ends up deteriorating. While, there arises a problem of deterioration in sintering when alkaline earth is increased in accordance therewith. Furthermore, Zn has a problem that it is easy to evaporate.
An object of the present invention is to provide a method of enabling to produce a dielectric ceramic composition preferably used as a dielectric ceramic composition for a multilayer chip capacitor wherein base metals like Ni, Ni alloy, etc. can be used for internal electrodes, segregation of different phases other than the main composition is controlled, the fine configuration of the dielectric is controlled, the capacitance-temperature characteristics satisfies the X8R characteristic, dielectric loss is small, permittivity and insulation resistance are high and the high temperature accelerated lifetime characteristic is excellent.
To attain the above object, according to the present invention, there is provided a method of manufacturing a dielectric ceramic composition, including at least
a main component constituted by barium titanate;
a second component as a sintering aid; and
other subcomponents;
comprising the steps of:
mixing in said main component at least part of other subcomponents except for the second subcomponent to prepare a pre-calcination powder;
calcining said pre-calcination powder to prepare a calcined powder; and
mixing at least said second subcomponent in said calcined powder to obtain a dielectric ceramic composition having ratios of the subcomponents to the main component of predetermined molar ratios.
In the present invention, preferably, said barium titanate as the main component has a composition expressed by BamTiO2+m where m is 0.995xe2x89xa6mxe2x89xa61.010 and a ratio of Ba and Ti is 0.995xe2x89xa6Ba/Tixe2x89xa61.010; and
said second subcomponent contains SiO2 as a main component and at least one type selected from MO (note that M is at least one type of element selected from Ba, Ca, Sr and Mg), Li2O and B2O3.
In the present invention, more preferably, said second subcomponent is expressed by (Ba, Ca)x SIO2+x (note that x=0.7 to 1.2). The second subcomponent is considered to work as a sintering aid.
When the second subcomponent has a composition expressed by (Ba, Ca)x SiO2+x (note that x=0.7 to 1.2), the ratios of Ba and Ca in the second subcomponent may be any or only one of the two may be contained.
In the present invention, the above pre-calcination powder is preferably calcined at a temperature of 700 to 1100xc2x0 C., more preferably at 800 to 1050xc2x0 C. Note that the calcination may be performed for a plurality of times.
It is sufficient that the pre-calcination powder is mixed at least the second subcomponent, and in accordance with need, the main component and part of other subcomponents may be further mixed and the composition of the finally obtained dielectric ceramic composition becomes within the range below.
According to a first aspect of the present invention, preferably,
said second subcomponent contains SiO2 as a main component and at least one type selected from MO (note that M is at least one type of element selected from Ba, Ca, Sr and Mg), Li2O and B2O3;
said other subcomponents comprises at least
a first subcomponent containing at least one type selected from MgO, CaO, BaO, SrO and Cr2O3;
a third subcomponent containing at least one type selected from V2O5, MoO3 and WO3; and
a fourth subcomponent containing an oxide of R1 (note that R1 is at least one type selected from Sc, Er, Tm, Yb and Lu); and
said calcined powder is mixed at least said second subcomponent and ratios of the respective subcomponents with respect to 100 moles of said main component are:
the first subcomponent: 0.1 to 3 moles,
the second subcomponent: 2 to 10 moles,
the third subcomponent: 0.01 to 0.5 mole, and
the fourth subcomponent: 0.5 to 7 moles (where, the number of moles of the fourth subcomponent is the ratio of R1 by itself).
In the first aspect of the present invention, preferably, said pre-calcination powder is prepared so that the molar ratio of components contained in said pre-calcination powder; (Ba+metal element of the first subcomponent)/(Ti+metal element of the fourth subcomponent) is less than 1, or (Ba+metal element of the fourth subcomponent)/(Ti+metal element of the first subcomponent) is over 1, and calcination is performed.
In the second aspect of the present invention, preferably,
said second subcomponent contains SiO2 as a main component and at least one type selected from MO (note that M is at least one type of element selected from Ba, Ca, Sr and Mg), Li2O and B2O3;
said other subcomponents comprises at least
a first subcomponent containing at least one type selected from MgO, CaO, BaO, SrO and Cr2O3;
a third subcomponent containing at least one type selected from V2O5, MoO3 and WO3; and
a fourth subcomponent containing an oxide of R1 (note that R1 is at least one type selected from Sc, Er, Tm, Yb and Lu); and
a fifth subcomponent containing an oxide of R2 (note that R2 is at least one type selected from Y, Dy, Ho, Tb, Gd and Eu); and
said calcined powder is mixed at least said second subcomponent and ratios of the respective subcomponents with respect to 100 moles of said main component are:
the first subcomponent: 0.1 to 3 moles,
the second subcomponent: 2 to 10 moles,
the third subcomponent: 0.01 to 0.5 mole,
the fourth subcomponent: 0.5 to 7 moles (where, the number of moles of the fourth subcomponent is the ratio of R1 by itself); and
the fifth subcomponent: 2 to 9 moles (where, the number of moles of the fifth subcomponent is the ratio of R2 by itself).
In the second aspect of the present invention, preferably, said pre-calcination powder is prepared so that the molar ratio of components contained in said pre-calcination powder; (Ba+metal element of the first subcomponent)/(Ti+metal element of the fourth subcomponent+metal element of the fifth subcomponent) is less than 1, or (Ba+metal element of the fourth subcomponent+metal element of the fifth subcomponent)/(Ti+metal element of the first subcomponent) is over 1, and calcination is performed.
In the second aspect of the present invention, it is preferable that the fifth subcomponent is contained in the pre-calcination powder at the time of preparing the pre-calcination powder. Also, when preparing the pre-calcination powder, it is preferable that the fourth subcomponent is not contained in the pre-calcination powder. Furthermore, when preparing the pre-calcination, it is preferable that the first subcomponent is always contained in the pre-calcination powder. Furthermore, preferably, the number of moles of the first subcomponent contained in said pre-calcination powder is smaller than the total number of moles of the fourth subcomponent and fifth subcomponent (note that the numbers of moles of the fourth subcomponent and fifth subcomponent are ratios of R1 by itself and R2 by itself, respectively).
According to a third aspect of the present invention, there is provided a method of manufacturing a dielectric ceramic composition, including at least
a main component constituted by barium titanate;
a second component as a sintering aid;
a sixth subcomponent containing CaZrO3 or CaO+ZrO2; and
other subcomponents;
comprising the steps of:
mixing in said main component said sixth subcomponent and at least part of other subcomponents except for the second subcomponent to prepare a pre-calcination powder;
calcining said pre-calcination powder to prepare a calcined powder; and
mixing at least said second subcomponent in said calcined powder to obtain a dielectric ceramic composition having ratios of the subcomponents to the main component of predetermined molar ratios.
According to a fourth aspect of the present invention, preferably,
said other subcomponents comprises at least
a first subcomponent containing at least one type selected from MgO, CaO, BaO, SrO and Cr2O3;
a third subcomponent containing at least one type selected from V2O5, MoO3 and WO3; and
a fourth subcomponent containing an oxide of R1 (note that R1 is at least one type selected from Sc, Er, Tm, Yb and Lu); and
said calcined powder is mixed at least said second subcomponent and ratios of the respective subcomponents with respect to 100 moles of said main component are:
the first subcomponent: 0.1 to 3 moles,
the second subcomponent: 2 to 10 moles,
the third subcomponent: 0.01 to 0.5 mole,
the fourth subcomponent: 0.5 to 7 moles (where, the number of moles of the fourth subcomponent is the ratio of R1 by itself); and
the sixth subcomponent: 0 to 5 moles (note that 0 is not included).
In the fourth aspect of the present invention, preferably, said pre-calcination powder is prepared so that the molar ratio of components contained in said pre-calcination powder; (Ba+Ca+metal element of the first subcomponent)/(Ti+Zr+R1) is less than 1, or (Ba+Ca+R1)/(Ti+Zr+metal element of the first subcomponent) is over 1, and calcination is performed.
According to a fifth aspect of the present invention, preferably,
said other subcomponents comprises at least
a first subcomponent containing at least one type selected from MgO, CaO, BaO, SrO and Cr2O3;
a third subcomponent containing at least one type selected from V2O5, MoO3 and WO3; and
a fourth subcomponent containing an oxide of R1 (note that R1 is at least one type selected from Sc, Er, Tm, Yb and Lu); and
a fifth subcomponent containing an oxide of R2 (note that R2 is at least one type selected from Y, Dy, Ho, Tb, Gd and Eu); and
said calcined powder is mixed at least said second subcomponent and ratios of the respective subcomponents with respect to 100 moles of said main component are:
the first subcomponent: 0.1 to 3 moles,
the second subcomponent: 2 to 10 moles,
the third subcomponent: 0.01 to 0.5 mole,
the fourth subcomponent: 0.5 to 7 moles (where, the number of moles of the fourth subcomponent is the ratio of R1 by itself);
the fifth subcomponent: 2 to 9 moles (where, the number of moles of the fifth subcomponent is the ratio of R2 by itself); and
the sixth subcomponent: 0 to 5 moles (note that 0 is not included).
According to the fifth aspect of the present invention, preferably, said pre-calcination powder is prepared so that the molar ratio of components contained in said pre-calcination powder (Ba+Ca+ metal element of the first subcomponent)/(Ti+Zr+R1+R2) is less than 1, or (Ba+Ca+R1+R2)/(Ti+Zr+metal element of the first subcomponent) is over 1, and calcination is performed.
In the fifth aspect of the present invention, it is preferable that the fifth subcomponent is always contained in the pre-calcination powder at the time of preparing the pre-calcination powder. It is also preferable that the first subcomponent is always contained in the pre-calcination powder when preparing the pre-calcination powder. It is further preferable that the first subcomponent, fourth subcomponent and the fifth subcomponent are always contained in the pre-calcination powder when preparing the pre-calcination powder. Furthermore, preferably, the number of moles of the first subcomponent contained in said pre-calcination powder is smaller than the total number of moles of the fourth subcomponent and fifth subcomponent (note that the numbers of moles of the fourth subcomponent and fifth subcomponent are ratios of R1 by itself and R2 by itself, respectively).
In a conventional method of producing a dielectric ceramic composition, barium titanate and additives were once mixed to produce mixed powder of the dielectric ceramic composition or a dielectric paste. In this method, however, segregation of additives, etc. (first to sixth subcomponents) occurs in the dielectric ceramic composition after firing and unevenness arises in the composition between respective crystals. Due to the segregation, permittivity of the dielectric and insulation resistance deteriorate.
According to the present invention, by mixing a main component with at least one of a first subcomponent, third subcomponent, fourth subcomponent, fifth subcomponent and sixth subcomponent excepting a second subcomponent and calcining, unevenness in the composition between respective crystal particles is suppressed, consequently, it is possible to produce a dielectric ceramic composition wherein deposition of a segregation phase is suppressed, a size of the segregation phase is controlled, the X8R characteristic is satisfied, the insulation resistance and specific permittivity are improved and excellent reliability is attained. These were found for the first time by the present inventors.
Also, according to the present invention, in the case of adding two kinds or more of rare-earth elements (fourth subcomponent and fifth subcomponent) in the dielectric ceramic composition, by always making the fifth subcomponent contained in the powder before calcination, it is possible to produce a dielectric ceramic composition having excellent reliability wherein deposition of a segregation phase is suppressed, a size of the segregation is controlled, the CR product and reliability are improved while the X8R characteristic is satisfied. This was also discovered for the first time by the present inventors.
Furthermore, according to the present invention, by making the sixth subcomponent contained in the dielectric ceramic composition, there are an effect of shifting the Curie temperature to the higher temperature side and an effect of flattening the capacitance-temperature characteristic. Also, there is an effect of improving the CR product and strength against direct-current insulation breakdown.
As explained above, since a capacitor produced by the method according to the present invention is able to satisfy the X8R characteristic of the EIA standard, it can be used under an environment such as being exposed to a high temperature as in an engine room of vehicles. Also, since a dielectric ceramic composition obtained by the production method according to the present invention does not contain element which is liable to be evaporated and dispersed, such as Pb, Bi and Zn, it is possible to be subjected to firing in a reducing atmosphere. Therefore, it becomes possible to use base metals, such as Ni and Ni alloy as an internal electrode so a lower cost can be attained.
Also, a dielectric ceramic composition obtained by the production method of the present invention satisfies the X8R characteristic in firing under a reducing atmosphere, has small deterioration in capacitance-aging characteristic and insulation resistance under direct-current electric -field application and excels in reliability, as well. Therefore, an effect can be expected as a method of suppressing deterioration of the temperature change rate in a high temperature range even if a multilayer capacitor is getting thinner.
Also, since a dielectric ceramic composition obtained by the production method of the present invention does not contain substances like Pb, Bi, etc., it is possible to provide a product which has a small adverse influence on an environment by disposal after using.
Also, in a production method according to the present invention, it is possible to realize a dielectric ceramic composition having a uniformed composition with a little different phases formed by segregation of additives, by which the permittivity and insulation resistance of the dielectric ceramic composition can be improved. Also, in the production method of the present invention, since it is possible to prevent structural default which accidentally occurs, a multilayer ceramic capacitor having a high reliability can be provided.
Furthermore, since it is possible to suppress segregation of different phases without changing the composition of additives, a dielectric ceramic composition wherein the capacitance-temperature characteristic satisfying the X8R characteristic can be easily produced.